1. Field of the Invention
The present invention relates to a multishaft electric motor for rotating a plurality of shafts in synchronism with each other and a positive-displacement pump which is combined with such a multishaft electric motor, and more particularly to a multishaft electric motor suitable for use with a rotary machine which is required to rotate two shafts synchronously in opposite directions, such as a two-shaft gear pump, a two-shaft screw pump, a two-shaft Roots blower, a two-shaft screw compressor, or the like, and a positive-displacement pump which is combined with such a multishaft electric motor.
2. Description of the Prior Art
Electric motors for use as driving means for driving pumps or the like include induction motors and direct-current motors. Generally, these motors have only one rotatable shaft.
FIG. 23 of the accompanying drawings shows in cross section a two-shaft rotary machine such as a Roots blower which is driven by an electric motor having only one rotatable shaft. The two-shaft rotary machine shown in FIG. 23 comprises a pair of juxtaposed rotors 32, 33 disposed in a housing 31 and having respective shafts 32a, 33a, and a pair of gears 34, 35 fixedly mounted on the shafts 32a, 33a, respectively, and held in mesh with each other. An electric motor 35 has a rotatable drive shaft 35a coupled coaxially to the shaft 32a of the rotor 32.
When the rotor 32 is rotated by the electric motor 35, the rotational drive power is transmitted from the rotor 32 through the gears 34, 35 to the other rotor 33. Therefore, the shafts 32a, 33a and hence the rotors 32, 33 are rotated synchronously in opposite directions.
Japanese laid-open patent publication No. 4-178143 discloses a two-shaft electric motor for rotating two shafts synchronously in opposite directions. The disclosed two-shaft electric motor is shown in FIGS. 24 and 25 of the accompanying drawings. As shown in FIGS. 24 and 25, two rotors 41, 42 with circumferential permanent magnets are disposed in a housing 40 such that the permanent magnets are held in contact with each other or are positioned closely to each other. The rotors 41, 42 are juxtaposed in a stator 44 mounted in the housing 40 and supported on parallel shafts that are rotatably mounted in the housing 40 by respective sets of bearings 45, 46. An array of armature elements 43 is mounted on an elliptical inner circumferential surface of the stator 44. The rotors 41, 42 jointly provide a magnetic coupling in confronting tooth-free regions thereof where unlike magnetic poles of the permanent magnets of the rotors 41, 42 face each other.
The two-shaft rotary machine shown in FIG. 23 suffers size and noise problems because the gears 34, 35 are required as timing gears for rotating the rotors 32, 33 synchronously in opposite directions.
In the two-shaft electric motor shown in FIGS. 24 and 25, an attractive force is developed due to the magnetic coupling between the rotors 41, 42 which are supported in contact with each other or with a small gap left therebetween. The attractive force thus developed is responsible for a radially unbalanced load imposed on the rotors 41, 42. To suppress an excessively large eccentric load applied to the bearings 45, 46 owing to the radially unbalanced load and to allow the rotors 41, 42 to rotate smoothly at high speeds, it is necessary to apply a certain magnetic attractive counterforce tending to cancel the magnetic attractive force acting between the rotors 41, 42. The armature elements 43 disposed on the elliptical inner circumferential surface of the stator 44 are not available for generating such a magnetic attractive counterforce because the armature elements 43 generate a rotating magnetic field for driving the rotors 41, 42. If the rotors 41, 42 are held in contact with each other, then no such magnetic attractive counterforce needs to be generated, but the contacting rotors 41, 42 are liable to produce an undue level of wear or noise.